1. Field of the Invention
The present invention relates to a process for forming a photoresist pattern which is suitable for the production of a semiconductor integrated circuit, and particularly to a process for producing a photoresist pattern according to which a superfine pattern is formed with a high precision.
2. Description of the Prior Art
With the progress of the high density of a semiconductor integrated circuit, the minimum size of a pattern of a circuit to be integrated is made more and more fine. Accordingly, there is a great demand for a technique of forming a fine photoresist pattern of about 1 .mu.m or a submicron or less with a high precision.
In general, the photolithography technique has heretofore been widely used as a means for patterning with a high precision by improving the resolution. Also the lithography technique using electron beams, X-rays, or ion beams as the ray source has been developed. However, the photolithography technique using light is advantageous when consideration is given to mass production, economy, and workability.
Various proposals have been made with respect to the process for forming a photoresist pattern according to the photolithography technique. According to, for example, a contrast enhanced photolithography technique (hereinafter abbreviated as "CEPL technique") as disclosed in IEEE Electron Device Letters, EDL-4, 1983, pp. 14 to 16 (literature I) and Journal of Vacuum Sciences and Technology, B3(1), Jan./Feb., 1985, pp. 323 to 326 (literature II), a photoresist pattern with a high resolution can be formed by the addition of a simple process, thus attracting attention.
The principle of the CEPL technique will now be explained with reference to FIG. 1.
FIGS. 1A to 1E are a process diagram for explaining the principle of the CEPL technique. Each figure is schematically drawn as a cross sectional view.
As shown in FIG. 1A, an underlying photoresist layer 12 to be subjected to patterning is provided on a silicon wafer (silicon substrate) 11. A thin photosensitive layer 13 (hereinafter often referred to as "CEL film") called a contrast enhancement layer is provided on the underlying photoresist layer 12. This CEL film is made of a material which at first shows high absorption of light with exposure wavelengths but shows lower absorption with a higher transmittance with an increase in the exposure does because it is bleached with light irradiation.
Since light reaches the region of a shadow of a mask 14 against a light source due to light diffraction and a focusing effect when light passes through a photomask 14, the light intensity distribution behind the photomask 14 assumes a state as shown in FIG. 1B. As a result, the contrast of a projected light image of the photomask is lower than the contrast threshold value of the underlying photoresist layer 12. Thus, photoresist patterning cannot be done with a sufficiently satisfactory resolution.
In this CEPL technique, selective light exposure is done by projecting a light image of the photomask 14 as shown in FIG. 1B on the underlying photoresist layer 12 through the CEL film 13. In this case, a portion 13a where the CEL film is bleached since the dose (exposure dose) is large, and portions 13b where the CEL film is not bleached since the dose is small are formed as shown in FIG. 1C. The transmittance of the CEL film 13 is partially largely changed by a difference in the degree of bleaching corresponding to the light intensity distribution. Thus, in an ideal case, the transmitted light intensity distribution assumes a state as shown in FIG. 1D. As a result, the light transmitted through the CEL film 13 has an enhanced contrast. Selective exposure of the photoresist layer 12 is done by irradiating the photoresist layer 12 with light having an enhanced contrast. By a subsequent development treatment, for example, a positive photoresist pattern 12a which is clear and sharp as shown in FIG. 1E is formed.
In the case of the above-mentioned conventional CEPL process, however, an additional step of removing the CEL layer prior to the development of the photoresist layer after light exposure is necessary as is apparent from FIG. 2.
This will be explained with reference to FIG. 2. An underlying photoresist layer 32 and a CEL film 33 are provided in this order on a substrate 31 (FIGS. 2A and 2B). Subsequently, irradiation with ultraviolet radiation is conducted through a photomask 34 to form exposed portions 32a and 33a and unexposed portions 32b and 33b in the same manner as described above (FIG. 2C).
After removal of the CEL layer 33 (33a and 33b) as shown in FIG. 2D, development of a photoresist layer 32 is done.
Specifically, development of the above-mentioned photoresist layer is conducted with an aqueous alkaline solution, while removal of the CEL is conducted with an organic solvent. Therefore, the above-mentioned additional step is necessary, thus making the process complicated.
According to the CEPL technique as disclosed in the literature II, the choice of a material for forming a CEL film 143 is important. The CEL film 13 as disclosed in the literature II is formed by dissolving diphenylamine-p-diazonium sulfate, which is a water-soluble diazonium salt, and polyvinyl alcohol as a binder polymer in water and applying the resulting solution onto an underlying photoresist layer 12.
However, in the process using the material as disclosed in the literature II, the diazonium salt is used by dissolving the same in water. Since this diazonium salt is decomposed by water, there is a problem that the CEL coating solution cannot be stably kept for a long time.
In a bleaching characteristic curve of this CEL film irradiated with light having a wavelength of 436 nm as shown in FIG. 3 in which the abscissa represents the exposure dose (mJ/cm.sup.2) and the ordinate represents the transmittance (%), the transmittance in an unexposed portion is as high as at least 60%. Therefore, the contrast enhancement effect is not so large that formation of a clear and sharp photoresist pattern cannot be expected.
Thus, a first object of the present invention is to provide an excellent process for forming a resist pattern which can solve the problems of complicatedness of the steps of the above-mentioned process and instability of the coating solution used for formation of the CEL film.
A second object of the present invention is to provide a process for forming a resist pattern which can solve the problem of the contrast enhanced effect.